1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a bow resistant fuel assembly structure for non-control rod locations of the reactor core.
2. Description of the Prior Art
The cores of nuclear reactors conventionally include a plurality of fuel assemblies. In a typical pressurized water nuclear reactor (PWR), all fuel assemblies are geometrically alike. Each fuel assembly includes a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles of the fuel assembly are secured to opposite ends of the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. The guide thimbles together with the top and bottom nozzles rigidly attached thereto compose the structural skeleton of the fuel assembly.
To control the fission process created by nuclear fuel contained in the fuel rods, typically a number of control rods are reciprocally positioned for movement in the guide thimbles of the fuel assembly. However, not all of the fuel assembly locations of a reactor core use control rods. Only about one-third of the fuel assemblies are in control rod locations. But since heretofore all PWR fuel assemblies have been constructed to be alike geometrically, this means that the fuel assemblies for control rod locations have been the same as those for non-control rod locations.
A departure from this prior practice of constructing all PWR fuel assemblies alike has been proposed recently. As described and illustrated in the patent application cross-referenced above, a separate fuel assembly design for non-control rod locations includes a bottom nozzle, a number of longitudinally extending structural members which contain a burnable poison and a top nozzle. It also includes a number of grids which are axially spaced and attached to the longitudinal structural members and support an array of fuel rods. The top and bottom nozzles are attached to the longitudinal structural members by screw thread connections or other suitable rigid attaching means. An instrumentation tube is located in the center of the assembly and is supported by the top and bottom nozzles and by the grids.
One important difference in this non-control rod fuel assembly over the conventional control rod fuel assembly lies in the design of the longitudinal structural members which interconnect the top and bottom nozzles to form the structural skeleton of the assembly. In the conventional PWR assembly, the structural members are the hollow guide thimble tubes which are open at the top and closed at the bottom (except for small holes for coolant flow). These tubes are positioned within the fuel assembly to align with the control rods. During reactor operation, the control rods move reciprocally in the tubes. On the other hand, in the non-control rod fuel assembly intended for use in non-control rod core locations, the structural member also in the form of tubes do not receive control rods. Therefore, different functional as well as structural use can be made of the tubes.
Functionally, this non-control rod structural member contains burnable absorber material. Burnable absorbers, such as a suitable compound of boron, are used in modern reactors to provide an additional means for controlling reactivity especially at the beginning of life of the nuclear fuel. Structurally, the elongated tube of the structural member is closed at each end by end plugs which are welded to the tube. The tube and end plug material is preferably Zircaloy-4. A spring holds the absorber material in place in the tube and provides a plenum for accumulation of helium gas which is released when a neutron interacts with a boron atom. To assemble the non-control rod structural members into the fuel assembly, the tubes must be empty and open at one end. After the grids are bulge fitted to the tubes, the absorber material and spring are loaded into the tubes and the remaining one end plugs welded in place. The fuel rods are then loaded and the top and bottom nozzles are bolted on.
In the non-control rod fuel assembly, there are eight absorber structural members whereas the conventional control rod fuel assembly has twenty-four guide thimbles. Thus, there are sixteen more fuel rods per non-control rod fuel assembly which has the benefits described in the above cross-referenced application.
The use of non-control rod fuel assemblies in PWRs having the design described above has created an opportunity to possibly overcome an important problem which has been present for a long time and affects the overall performance of PWR fuel assemblies: fuel assembly bow. There appears to be a definite relationship between the magnitude of fuel assembly bow and compressive stresses in the guide thimbles. Unfortunately, there is no readily apparent method of appreciably reducing the compressive stresses in the guide thimbles of control rod fuel assemblies which are in control rod core locations. However, for fuel assemblies in non-control rod core locations and designed as described above, an opportunity would appear to exist to find a way of greatly reducing the compressive stresses in the longitudinal structural members.